1. Field of the Invention
The present invention relates to solar stills and more specifically to the use of a specialized high rate falling film heat exchanger/evaporator submerged within a solar energy collection pond which utilizes the salt gradient principle. The evaporator component of the invention also has applicability submerged within other liquid bodies which contain sufficient heat for the desired use. The invention has particular application in geographical areas of high insolation, inadequate water supplies and relatively inexpensive land. The high rate solar still of the present invention may be used to purify sea water, brackish water, fresh water containing unusually high fouling impurities, and for the recycling of irrigation water; especially in those cases where alternative water treatment processes, including membranes, require prohibitively expensive pretreatment techniques in order to operate.
2. Description of the Prior Art
The art of producing drinking water from salt water by means of solar distillation is well known. Most designs of solar stills consist of floating or semi-submerged apparatus enclosed by material transparent to solar radiation wherein a pool of salinous water is allowed to heat up and vaporize. Condensation of the vapor is frequently accomplished by routing the vapor through a deeper level of seawater. Attempts have been made to improve the efficiency of solar energy collection by using lens and solar tracking devices. Examples of prior art are disclosed in U.S. Pat. Nos. 4,325,788, 4,276,122, 4,219,387, 4,172,767, 3,986,936, 3,703,443, 3,408,260, 3,357,898 and 4,151,046.
Systems have heretofore been constructed wherein flat plate solar collectors feed hot water to conventional multistage flash distillation plants. A demonstration scale system of such a system has been in operation in Mexico for several years. Another approach has been to use photovoltaic cells to generate electricity that operates a membrane separation desalination process.
Most of the prior art dealing with solar energy collection ponds, consists of articles published in various trade and scientific journals and, in particular, recent research conducted and published on salt gradient solar ponds by the U.S. Departments of Interior, Energy and Agriculture. In U.S. Pat. No. 4,110,172 an enclosed solar collection pond is disclosed containing salinous water with a depth of one to ten inches. This invention is focused on the rapid heating of the shallow pool of water and the recirculation of the heated air in order to raise the efficiency of the process.
A ditch similar in configuraton to the solar pond utilized in the present invention is described in U.S. Pat. No. 4,141,798. However, the solar still arrangement as described therein is substantially similar in principle to the above mentioned prior art which uses a flexible transparent plastic enclosure operating on a relatively shallow pool of salinous water, i.e., the greenhouse effect.
Applicant is not aware of any prior art which uses a salt gradient solar pond, with its superior temperature elevation capability and heat storage capability in the absence of insolation and/or in the cold season, in combination with a submerged evaporator device specially designed to take advantage of this stored heat in a space much smaller than required by shallow, non-salt gradient ponds, and constructed in such a way as to minimize the consequences of the corrosive environment.
The horizontal tube falling film evaporator has been in commercial desalination service since the late 1960's, in the single effect vapor compression mode, and more recently, in the multiple effect configuration using generated steam as an energy source. This evaporator design has also been used in connection with solar ponds and waste heat with the intent to produce power. However, in each such case the evaporator consisted of a conventional shell and tube arrangement situated external to the heat laden liquid body. In the recent past, solar ponds have been used to heat water flowing through submerged tubing for space heating purposes without severe corrosion problems.
A rotating disk distillation device is described in U.S. Pat. No. 3,764,483 which operates on the principle of a hydrodynamically applied thin film relying on the wiping action of a flexible blade. A multiple effect version of this device has been proposed for use with a salt gradient solar pond. However, the device was proposed to be situated external to the pond and is not suitable for submerged operation, nor is the evaporation principle the same as the present invention, i.e., wiped film disk versus horizontal tube falling film.
Accordingly, there appears to be no disclosure in the prior art of an evaporator submerged within the heat source, with exposed tube sheets, so as to reduce construction costs, reduce ambient heat loss, eliminate pumping of heating medium, and that rotates to (1) maximize the falling film heat transfer effect, (2) repeatedly expose the concentrate to heat transfer surface, (3) periodically submerge a large proportion of the heat transfer tubes to reduce scaling and (4) induce flowthrough of the heating medium, and that is constructed with a minimum use of welds to reduce corrosion. This combination of features cooperate to maximize the efficiency of the distillation process while minimizing space requirements compared to the prior art.